The present invention relates to the maintenance of sufficient water depth in harbors and navigation channels, and more particularly, to a method and apparatus for eliminating the need for periodic dredging of such waterways by passively resuspending sediments transported to the waterways by natural currents.
Most of our present day harbors were sited several hundred years ago in the embayments and quiet waters around drowned river mouths. Although the natural water depth in these harbors was adequate for the 6-m-draft wooden sailing ships of that era, ships' drafts have since more than doubled. Indeed, a fully loaded supertanker, aircraft carrier, or nuclear submarine may draft almost as much as 14 m.
Few of the historic harbors provide or maintain that much deep water through natural circulation and scour; those that do, such as in San Francisco, Puget Sound, or San Diego, still have local shallows, shoals, and inlet bars requiring artificial deepening. Other historic harbors were once natural deepwater harbors, but are no longer. An example is Charleston, S.C., where upstream the Cooper River has been diverted for a hydroelectric plant, resulting in as much as 5 m of annual shoaling in Charleston Harbor. The sediments are, of course, the continual by-product of erosion and weathering; their transport pathway down the rivers to the sea is intercepted by the artificially deepened harbors.
With the advent of steam power and the first deepdraft iron-hulled ships, the first suction dredges also appeared, and it became easier to dig than to relocate the harbors. Deepening our harbors by overdredging has diminished the bottom velocities and stresses caused by natural circulation and has thereby retarded the transport of sediment bound for the sea. Thus the deeper a harbor is dredged, the more rapidly subsequent sedimentation acts to refill it.
Also, after 150 years of dredging, places to dump all the sediments are scarce. The problem is compounded by recent environmental constraints since it has been learned that these sediments tend to concentrate heavy-metal toxins, halogenatd hydrocarbons, pesticides, and huge amounts of anaerobic bacteria. The contaminants are released back into the water by the bottom agitation of dredging activity. These factors combine with rising energy costs to render continual maintenance dredging too expensive or too harzardous to the environment.